Using the same source file from other source files

I know I saw the answer to this in a FAQ somewhere in the past week or two,
but now tat I need to know it, I can't find which FAQ it was in.

I have a program that's grown to several files and I was compiling it by
just using #include where necessary and everything was working okay.  Now
I'm changing over to make to manage it better and reduce compiling time.

I have some files with functions that are used by the "main" file and by
other files.  For instance, I have main.cpp that includes a.cpp and b.cpp
and c.cpp.  I also need c.cpp in a.ppp.  If I don't use '#include "c.cpp"'
in a.cpp then it won't create an object file, but if I do use that, then I
also need it in main.cpp and at that point I have problems with everything
in c.cpp being defined more than once.

How can I make sure several files can access the functions in a separate
file without getting repeated definitions?

Thanks!

Hal